Automotive electrical systems are required to supply electrical power to a variety of devices within a vehicle. These devices typically include an electric starter motor, the engine ignition system, an electronic control unit (computer), headlights, and a variety of accessories. Electrical power is supplied to these devices (loads) by a battery, wherein the most common type is a 12V lead-acid battery. The battery is typically the primary or only source of electrical power when the vehicle's engine is not running, whereas the battery and an alternator typically both supply electrical power when the engine is running.
The alternator converts mechanical power generated by the engine into electrical power that is used both for meeting the demands of the various electrical devices and for charging the battery. In order to maintain an adequate charge state in the battery and to avoid damaging the battery (e.g., overcharging) or the various electrical devices, the alternator must generate an output voltage in a fairly narrow range. To support a typical 12V battery and an associated electrical system, the alternator should supply an output in the range of 14.0 to 14.6 V, with an ideal voltage of about 14.2V. The voltage output from the alternator is controlled using some type of voltage regulation.
The output voltage for most modern alternators is regulated by controlling the amount of current flowing through the windings in the rotor of the alternator. This current, in conjunction with the rotation speed of the rotor, determines the induced current flowing through the windings of the stator which, after rectification, produces the alternator's output voltage. Voltage regulation is accomplished by sensing the output voltage from the alternator, and using the sensed voltage to determine an appropriate excitation current to provide to the rotor.
Control of the excitation current provided to the rotor is accomplished via appropriately switching a supply voltage (e.g., the battery voltage) that provides current to the rotor. This switching is typically performed by a power transistor, which is controlled by an excitation control signal. The excitation control signal may be generated by a controller within the alternator, in conjunction with voltage regulation performed by the controller and/or by an electronic control unit (ECU) located outside of the alternator.
A potential safety issue arises if a fault causes a “full field” condition in the alternator. This occurs when the supply voltage becomes connected directly to the rotor such that the rotor's excitation current is no longer limited by the controller within the alternator and the associated voltage regulation. Such a fault leads to an uncontrolled alternator voltage output, which is likely to reach excessive levels that may damage the battery (by overcharging it) or other devices connected to the electrical system.
One solution to the above problem is to, upon detection of a “full-field” condition, disconnect the alternator output from the battery and the other components of the electrical system. However, the alternator outputs relatively high current, e.g., in the range of 50-200 amperes, which makes the switching of this output unfeasible and/or prohibitively expensive.
Another proposed solution implements a safety function that is implemented by interrupting the control signal, e.g., from the controller of the alternator to the switch controlling the rotor excitation current. Such a solution addresses faults in the controller itself, faults caused by inputs to the controller (e.g., an incorrect sense voltage), or a fault in the signal controlling the excitation current control switch.
However, the above remedy fails to address the situation in which the full-field condition is caused by a short occurring around the excitation control switch, i.e., the situation wherein the supply voltage is electrically shorted directly to the rotor input. A fault protection circuit and method for addressing a full-field condition caused by such a fault is desirable.